1. Field of the Invention
The present invention relates to a method of removing glass adhered to sintered objects of inorganic powder such as silicon nitride powder, silicon carbide powder or the like during hot isostatic pressing process.
2. Description of the Prior Art
In the manufacturing of sintered objects of silicon nitride, a preformed body of silicon nitride powder coated with high-melting glass is subjected to hot isostatic pressing during which the preformed powder body is isotropically pressed with a pressure medium such as inert gas under pressure at a sintering temperature. The hot isostatic pressing is advantageous to manufacture sintered objects of complicated shapes having the same strength in all directions at a high sintering temperature. During the hot isostatic pressing process, however, the glass used as a sealing material is inevitably adhered to recesses of the sintered powder body. To remove the adhered glass from the sintered powder body, there have been proposed a hammering method by applying a mechanical external force to the sintered body, a method of crystallizing the glass adhered to the sintered powder body under control of its cooling temperature after sintering to facilitate removal of the glass, and a method of vaporizing the pressure medium melted in the layer of glass for foaming the glass and cooling the foamed glass.
In the hammering method, it is required to mechanically apply a great external force to the sintered powder body for removal of the glass adhered thereto. This results in an increase of the manufacturing cost of this kind of sintered objects. The second crystallizing method is effective only in use of special glass such as silica glass. In the third method, foaming of the glass is greatly influenced by a condition of the hot isostatic pressing and the kind of the glass. For the foregoing reasons, it is very difficult to remove the glass from the sintered powder body at a low cost.